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Mars Express radar reveals complex structure in ionosphere of Mars

30 November 2005

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The principle of radar science used by MARSIS is based on the detection of radio waves reflected by boundaries between different materials. By analysis of these echoes, it is possible to deduce information about the kind of material causing the reflection, such as estimates of its composition and physical state.

Different materials are characterised by their ‘dielectric constant’, that is the specific way they interact with electromagnetic radiation, such as radio waves. When a radio wave crosses the boundary of different layers of material, an echo is generated and carries a sort of fingerprint from the specific materials. The MARSIS antenna booms here are seen receiving reflected signals. The red dotted line denotes the top of the ionosphere of Mars.

Credits: ESA (Animation by AOES Medialab)

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An animated sequence of MARSIS ionograms (typical products of MARSIS radar sounding of the ionosphere), covering an entire pericentre pass on 14 August 2005. MARSIS detects electron cyclotron echoes, and derives the related frequency. This depends on the magnitude of the surrounding magnetic field. When the spacecraft passes over areas of higher magnetic field strength, the frequency changes. In this animation, we can hear it in the form of changing sound.

Credits: ASI/NASA/ESA/Univ. of Rome 'La Sapienza'/Univ. of Iowa/JPL

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MARSIS ionospheric sounding data are shown in this plot, called an 'ionogram'. The horizontal axis is the frequency of the pulse and the vertical axis is the time delay after transmitting the pulse, plotted positive downward and converted to apparent range to the reflection point.

The intensity of the received signal at any given frequency and apparent range is indicated by the colour, with dark blue being the least intense and green the most intense. The green echo at an apparent range of about 800 km from 2.5 to 5.5 MHz is the reflected signal from the surface of Mars.

The curved bright green feature with an apparent range varying from about 600 to 750 km at frequencies from about 0.7 to 1.8 MHz is the echo from the top side of the ionosphere.

A second echo in the ionosphere, at an apparent range of about 100 km is labeled 'oblique ionospheric echo'. Such echoes are believed to come from distorted structures in the ionosphere caused by the magnetic fields in the crust of Mars.

Credits: ASI/NASA/ESA/Univ. of Rome/Univ. of Iowa/JPL

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